1. Field of the Invention
The present invention provides a thick steel plate having excellent strength and toughness and furthermore a thick steel plate devoid of material anisotropy and having excellent brittle crack propagation stop characteristics.
2. Description of the Related Art
Properties of a thick steel plate used as structural members or for other purposes are determined by its chemical components and heat treatment. Recently, production of a thick steel plate having excellent strength and toughness has become possible by a controlled rolling method predominantly comprising rolling at a low temperature and by an accelerated cooling method which conducts cooling in succession to rolling. Such production techniques are described in Japanese Examined Patent Publication (Kokoku) Nos. 49-7291, 57-21007, and 59-14535.
In controlled rolling in general, austenite grains are made fine in a high temperature region by recrystallization and further drawn sufficiently under the non-crystallized state in a low temperature region to obtain fine ferrite by transformation in a subsequent accelerated cooling process.
However, when such rolling in the recrystallization temperature region and rolling in the non-recrystallization temperature region are combined, the problem remains that a long waiting period is necessary for the drop of the rolling temperature and thus the productivity is remarkably impeded. Another problem resides in that the effect of rolling is lost during the period from the end of rolling in the non-recrystallization temperature region to the start of accelerated cooling (mainly because of the decrease of the dislocation density introduced by rolling), and the effect of rolling in the non-recrystallization temperature region cannot be exploited fully.
Still another problem resides in that when rolling is finished in the non-recrystallization temperature region, the rolled aggregate texture is transferred as such to the texture after rolling and material anisotropy increases. When rolling is carried out in the recrystallization temperature region in order to prevent this material anisotropy, there occurs the problem that since the rolling temperature is high, the grain growth after recrystallization is so fast that the crystal grains become coarse. When rolling is finished in a temperature region as low as possible within the range in which recrystallization can take place, however, partial recrystallization is likely to occur and duplex grains develop and cause deterioration of the material. Accordingly, there is a limit to the lowering of the rolling temperature.
The structural members must have excellent brittle crack propagation stop characteristics as one of the required characteristics.
As one of the metallurgical factors that affect brittle crack propagation characteristics when brittle breakdown occurs, it is well known that fine granulation of the crystal grains improves the brittle crack propagation stop characteristics. For this reason, a large number of attempts have been made in the past to make the crystal grains finer, and a thick steel plate having fine crystal grains has become available by a controlled rolling method in a low temperature region or by an accelerated cooling method which conducts cooling in succession to rolling, for example. Such a technique is described in Japanese Examined Patent Publication (Kokoku) Nos. 49-7291, 57-21007, and 59-14535.
Fine granulation of the crystal grains of a plate surface portion is extremely effective for improving the brittle crack propagation stop characteristics. Therefore, Japanese Unexamined Patent Publication (Kokai) No. 61-235534, Japanese Patent Application No. 4-67514, and Japanese Patent Application No. 4-67515 disclose a fine granulation method which combines water cooling during rolling with rolling. All of these related art references disclose the fine granulation method which cools the surface layer portion of the plate with water during rolling so as to bring the texture into an austenite-ferrite dual phase state or a ferrite single phase, conducts rolling during the process in which the temperature of the surface portion of the plate recuperates and rises by heat transfer inside the plate, so as to make the ferrite crystal grains fine and to introduce a rolling strain into the austenite, and eventually makes the crystal grains of the surface portion of the plate fine after transformation.
However, the method described in Japanese Patent Application Nos. 4-67514 and 4-67515 and Japanese Unexamined Patent Publication (Kokai) No. 59-182916 essentially stipulates the requirement that the highest arrival temperature of the plate surface portion by recuperation after water cooling be less than an Ac.sub.3 point to make the crystal grains of the plate surface portion finer. Accordingly, the machined texture of the ferrite remains and the toughness drops.
On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 61-235534 prevents residual machined texture from occurring by stipulating the essential requirement that the temperature of the plate surface portion after water cooling be recuperated to a point above the Ac.sub.3 point by heat transfer inside the plate. However, since the recuperative temperature exists on a higher temperature side, the resulting crystal grains become greater than those obtained by the method of Japanese Patent Application Nos. 4-67514 and 4-67515, and the brittle crack propagation stop characteristics, too, tend to be inferior.
There are various hot machining methods, and bending is one of them. A strain can be imparted without changing the plate thickness by repeating bending. However, there remains the problem that the strain imparted by bending is generally great in a plate surface portion and is not sufficiently imparted in the center portion in the direction of the plate thickness. For this reason, bending is employed primarily for improving the flatness of the plate but is not used for improving the material properties, in many cases. Japanese Examined Patent Publication (Kokoku) No. 1-16210 discloses a technology which improves a drilling ratio by hot molding a fine grain ferrite, but this reference does not describe the crystal condition between the strain during hot molding and the crystal grains, and so forth.